1. Field of the Invention
The present invention relates to an oxide high-temperature superconductor, and more particularly, to a Bi system copper oxide (Bi-Sr-Ca-Cu-O system) superconducting thin film and a method of forming the same.
2. Description of the Related Art
Attempts have been made to apply superconductors to devices or wirings, using the superconduction properties thereof (e.g., zero electric-conductivity), but since critical temperature (Tc) of conventional superconductors is very low, they can be used only in a liquid helium (He). Recently developed oxide superconductors, however, have a relatively high Tc, and thus can be used in a liquid nitrogen (N). Furthermore, an oxide high-temperature superconducting thin film can be formed by depositing oxide on a substrate using atomic layer control, and such a film formation has been studied and attempted (cf., Tomoji KAWAI: "Design and Synthesis of High Tc Superconducting Superlattice by Successive Deposition Method", Bulletin of the Japan Institute of Metals, Vol. 29, No. 9, (1990), pp. 733-739, and J. N. Eckstein et al., "Epitaxial Growth of High-temperature Superconducting Thin Films", J. Vac. Sci. Technol. B7 (2), 1989, pp. 319-323).
For example, a Bi system copper oxide superconducting thin film composed of Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x is illustrated in FIGS. 5 and 6 (page 736) of the above-mentioned publication of Tomoji KAWAI, and in FIG. 1 (page 552) of H. KOINUMA and M. YOSHIMOTO: "Development of Superfine Ceramics Chemical", Chemistry, Vol. 44, No. 8, (1989), pp. 552-553.
In Bi system copper oxide superconducting thin films, it is known that a film of Bi.sub.2 Sr.sub.2 CuO.sub.6 (n=1, without Ca) is flat. When "n" is 2 or more (i.e., Ca is contained in the oxide superconducting thin film), how ever the flatness of the film becomes poor and the unevenness (irregularity) of the film becomes pronounced, and depending on circumstances, precipitates of Cu and Ca appear, and as a result, the critical temperature (Tc) of the superconducting thin film is lowered. Furthermore, when such films are formed by a layer-by-layer process at an atomic layer level, it has been found that Ca grows three-dimensionally rather than two-dimensionally by an in-situ observation using RHEED (reflection high energy electron diffraction method) during the film deposition (growth). Once the uneven CaO molecular layer is formed, the CuO molecular layer deposited thereon is made uneven by the uneven surface of the CaO molecular layer, and as a result, the obtained superconducting thin film is also uneven as a whole.